explosion/selecting everything inside a sphere

Questions & Answers
1

let's say I have an explosion.

there's a centre of explosion (Vector3, point in space) and there's a sphere around it, with certain radius.

how to detect which objects in the scene are "inside" this sphere's radius?

3

Use Physics.OverlapSphere (as long as the objects you're interested in have colliders).

2

Basically:

if (Vector3.Distance(enemy.transform.position,explosion.transform.position)<radiusOfSphere){
    // do stuff here
}

This will measure from the pivot of the target object, so you might not get a true if its center is outside of the sphere (even though part of it might be inside). You could also instantiate a spherical object and check for collisions (if everything else has colliders). It depends on how exact you want your explosion effect...